The water lines in my TT are all PEX, same as my house, so pressure isn't an issue.
As Oakman said, shutoff valve (though I have seen shower heads with cutoffs in the past) and cost. I suspect another factor might be room. In a stick and brick you can move back from the shower head to get your "toes" wet. Campers don't have that kind of room, so you have to be able to move the shower head to where you need it.
2014 Coachmen Catalina 253RKS
2016 Keystone Summerland Mini 1750RD
2012 F250 Lariat 6.2L Crew Cab FX4
Reese dual cam SC/WDH